Coherent ultraviolet (UV) radiation has been widely used in material processing including micromachining, instrumentation, medical therapy/treatment/implant, and many other important applications. Medical applications are essentially based on interaction of UV light with human tissues. Laser material processing has become a key enabling technology in the ever-continuing trend of miniaturization in microelectronics, micro-optics, and micro-mechanics. Laser material processing is primarily based on conversion of the radiation energy into heat. During the process, the material experiences phase transition, from solid state to gaseous (plasma) state. The accompanying high temperature generates a Heat Affected Zone (HAZ), which is the source of many undesired machining properties, such as poor surface finish, internal remaining stress, etc. This problem becomes even more serious for high-energy conditions unless the laser is operated at short pulses. When the intense laser-material interaction occurs in a very short time period (maximum 20 ns, best less than 1-ns), the heat is supplied to the work piece so fast that losses due to heat conduction during drilling, cutting, welding, or marking are negligible. Ideally, only the material to be removed absorbs the incident high intensity laser energy, while the other parts experience little influence.
Current sources of coherent UV light are not entirely satisfactory, each has some unfortunate drawbacks. Excimer lasers can directly produce output beams with high average powers, but require toxic, corrosive halogen gases for operation, which necessitates gas processing, storage and circulation technologies. These lasers are bulky, complex, potentially hazardous, and expensive. Furthermore, they cannot operate at high pulse repetition rates, and their beam transverse mode is quite far from TEM00. Ion lasers including frequency-doubled ion lasers are available at a number of wavelengths in the visible and UV region. However, they are inefficient, have high operating costs and short lifetimes. Dye lasers are impractical for large-scale industrial production since they require frequent changes of the liquid dye solution to maintain operation. In comparison with excimer lasers, solid-state UV lasers have the advantages of compact structure, maintenance-free, improved reliability, and can be operated at much higher repetition rates with much less energy fluctuations.
Solid-state UV lasers typically employ nonlinear optical processes for frequency conversion. For example, a wavelength converter disclosed by Masuda et al. in U.S. Pat. No. 6,249,371 comprises two laser sources. Two nonlinear optical processes are successively applied to the first laser source for fourth harmonic generation (FOHG). The converted wavelength is then mixed with the wavelength of the second laser source for sum frequency generation (SFG). Alternatively, in U.S. Pat. No. 6,373,869, Jacob teaches an optical system for producing UV radiation comprising a laser source that emits the fundamental wavelength, an optical parametric oscillator (OPO), a frequency doubler, and a mixer.
In U.S. Pat. No. 6,031,854, Ming teaches a diode pumped cascade laser for UV generation. A first solid-state laser is Q-switched to produce laser pulses of shorter than 50 ns with multiple-millijoule energy. This laser is further frequency-doubled to a wavelength near 530 or 660 nm. A second solid-state laser is pumped by the first solid-state laser and is then gain-switched to produce laser pulses of about 1-ns with energy of about 1 mJ. The second solid-state laser is further frequency-converted via fourth or fifth harmonic generation to produce UV output around 210 nm.
In US Patent Application No. 20070177638, Seelert, et al. teaches a solid-state laser based on a praseodymium-doped crystal gain-medium pumped by frequency-doubled, optically-pumped external-cavity surface-emitting semiconductor laser to produce laser in the visible spectral range. In particular, a Pr:YLF laser that produces a fundamental wavelength at 522 nm is investigated. After frequency doubling, a UV light of around 261 nm is obtained.
An alternative approach is described by Owa et al. in U.S. Pat. No. 6,088,379, wherein a Ti:Sapphire laser generating a wavelength of approximately 707 nm is used as the first light source and a frequency-quadrupled neodymium-doped solid-state laser generating wavelength near 266 nm is used as the second light source. The two laser beams are then sum-mixed in a nonlinear crystal to produce UV wavelength of approximately 193 nm.
Titanium-doped sapphire (Ti:sapphire or Ti:Al2O3) is a solid-state lasing material having a broad vibronic fluorescence band. This spectroscopic property allows tunable laser output between 670-1070 nm with the peak of the gain curve around 800 nm. With fourth harmonic generation, it is possible to produce laser output at a wavelength below 200 nm. In US Patent Publication No. 20050094682, Tulloch, et al. discloses a Ti:sapphire-based laser system that produces tunable UV output between approximately 187 and 333 nm. Ti:sapphire exhibits a broad absorption band, located in the blue-green region of visible spectrum with a peak around 490 nm. It is commonly pumped by another laser, e.g., argon ion laser, copper vapor laser, frequency-doubled diode pumped Nd:YLF laser or Nd:YAG laser. This reduces the overall efficiency and makes the system complicated and expensive.
While there is no commercially available edge-emitting laser diode that produces laser wavelength around 490 nm, such wavelengths can be produced by GaN or GaInN light emitting diodes (LED). Based on amplified spontaneous emission (ASE), LED radiation is incoherent and quasi-monochromatic, typically with a bandwidth around a few nanometers. Quite a few lasing gain media, in particular those with broad absorption spectra, can be spectrally matched with LED emission.
Compared to semiconductor laser devices, LED normally has wider divergence angles, around ±20°. For efficient and uniform injection of the pump energy into lasing gain media, US Patent Application No. 20050201442, entitled “Solid-State Lasers Employing Incoherent Monochromatic Pump”, discloses an apparatus, wherein a diffusion pump chamber effectively surrounding the gain medium is employed to enhance effects of the pump light on the gain medium. With this apparatus, laser output at a wavelength below 800 nm can be directly produced from a solid-state system.
Efficient nonlinear processes require stable wavelength and intensity, as well as TEM00 beam quality. A common approach to stabilizing laser wavelength is injection seeding. A basic requirement for effective injection seeding is that resonance between the slave modes and the photons from the master must be kept whenever the oscillation modes are established. Conventionally, the master-slave resonance is based on stabilized mode frequency of the seed laser (master), active control of the resonance wavelength or longitudinal modes of the seeded laser (slave), and locked phase angle between the injected and output signals. These technologies require complex and expensive systems.
A novel technology that employs continuous wavelength sweeping for master-slave resonance was disclosed by Luo et al. in United States Patent Publication No. 20060215714, entitled “Injection Seeding Employing Continuous Wavelength Sweeping for Master-Slave Resonance”. By intentionally varying the seed laser drive current at a radio frequency (RF), the wavelength emitted from the seed laser continuously sweeps over a range covering one or more longitudinal modes of the slave oscillator. The swept spectrum has a stabilized profile with stabilized central wavelength. Active cavity length control and phase locking between the seeder and the seeded laser thus are not needed.
Many applications require reliable mid to high average power UV including DUV (wavelength <200 nm) lasers with short pulse width (<10 ns, ideally <1 ns) and high repetition rate (>100 kHz). To date, no commercially available lasers can meet all these requirements in an efficient and cost-effective manner.